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Monkeys Suffer Human-Like Depression

This article was originally posted on RealClearScience.

It’s no secret that depression is a major problem worldwide. A map published by the Washington Postdepicts the prevalence of depression around the world. Every country on Earth, rich or poor, has citizens who experience depression. Coping with sadness is a part of the human condition, but why some of us suffer in near perpetuity is not understood. Decades of research have produced little more than expensive pills that work only slightly better than a placebo.

There are many possible reasons why depression research has produced such disappointing results. One of them may be the fact that most depression research utilizes rodents in conditions that do not properly imitate human life. For instance, forcing a mouse to swim stresses it out, but this hardly replicates the social and cultural conditions in which humans develop depression. Besides, stress and depression, while related, are not the same.

In an effort to address this problem, a team of researchers at Chongqing Medical University, in collaboration with Wake Forest University, examined depression in cynomolgus macaque monkeys living in social colonies. Each colony lived in its own enclosure and contained two males and roughly 20 females (which reflects the natural male-female ratio), as well as their offspring. Fifty-two such colonies were observed, and a total of 1,007 female monkeys were screened for depression. Twenty females exhibiting frequent depression symptoms (e.g., lack of interest in eating, mating, and grooming) were selected, and they were matched with 20 healthy controls as well as with 10 monkeys that exhibited symptoms of depression due to experimental social isolation.

The authors found that the naturally depressed and the isolated monkeys exhibited some similar behaviors, specifically nursing infants for shorter durations and sitting on the floor for longer durations. Most importantly, just like what occurs among humans, they found a whole host of metabolic differences between the naturally depressed monkeys and the healthy controls.

The authors conclude that their model of depression using cynomolgus macaques mimics “real life” with all its psychosocial stressors and, hence, is superior to other models. They are probably right, but there are some major caveats.

First, the metabolic profiles of depressed monkeys were not identical to those of humans. Second, the authors failed to reverse symptoms of depression in the macaques when they were treated with ketamine, an antidepressant. Third, monkeys are expensive and research that utilizes them will tend to have smaller sample sizes. It would be incredibly inconvenient, for instance, to raise 100 monkeys with the hope that 5 of them might develop depression. To increase the number of test subjects, researchers would likely have to force monkeys into social isolation, which would be ethically dubious.

Having said that, studying depression in laboratory animals is important in its own right. The authors show rather convincingly that depression occurs naturally in macaques, and the disease manifests in a way that would be familiar to humans. Not only does this have ethical implications for primate research, it also raises questions about the reliability of biomedical data collected using depressed animals. Surely, such physiological changes must be factored in.